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circles have a flat spot
« on: September 02, 2017, 01:20:40 AM »
hi Everyone,
only new at cnc plasma, purchased a second had home made cnc, using logitrol controller sheetcam and Mach3 software, finally ironed out firing problems etc.

but when I cut circles I have a flat spot "like a flat tyre" parallel to x axis.

anyone know if it could be a software problem of the motor on the Y axis may have a problem. any thoughts would be appreciated?

cadmec.

Online Tweakie.CNC

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Re: circles have a flat spot
« Reply #1 on: September 02, 2017, 02:03:49 AM »
Check for lost motion (backlash) on your Y axis (this could be caused by a loose coupling, loose grub screw, etc.).

Tweakie.
Success consists of going from failure to failure without loss of enthusiasm.  Winston Churchill.
Re: circles have a flat spot
« Reply #2 on: September 02, 2017, 02:11:43 AM »
thanks Tweakie, will look for loose items
Re: circles have a flat spot
« Reply #3 on: September 02, 2017, 05:20:05 PM »
Hi,
what sort of mechanism moves the axes? Are they rack and pinion gears or ballscrews or leadscrew and nuts? How are the steppers hooked
up to each axis?

I go with tweakie, there is some lost motion in one or more axes. Can you measure any backlash in the Y axis? This is a common and pretty much
unavoidable problem with rack and pinion driven axes.

Craig
My wife left with my best friend...
     and I miss him!
Re: circles have a flat spot
« Reply #4 on: September 02, 2017, 05:27:56 PM »
hi Craig,
mmm on the Y axis - stepper motor with small gear the drives on a geared length bolted on to the gantry.

if that makes sense.

frank
Re: circles have a flat spot
« Reply #5 on: September 02, 2017, 06:38:41 PM »
Hi Frank,
yep, sure does. Its called rack and pinion and is very common in large plasma tables.

What you need to do is measure its backlash. I would suggest you use a dial gauge and see if you can measure any free movement by pushing the
gantry fore and aft. Ideally there would be no backlash but there will be. Some setups have the servo and pinion floating under spring pressure to
increase the engagement between the pinion and its rack. Its a good idea until the pinion gets worn then the peak of the teeth bottom in the groove
of the rack and will not allow any further engagement. This problem requires a good eye for mechanical detail to detect.

There are other methods for achieving low lash, variations on a split pinion, where two pinions along side each other engaged in the same rack but
with one pinion displaced, radially usually under spring pressure, by a fraction of a degree to effectively 'increase the width of the tooth to fill the gap'.
Clever, expensive and prone to wear.

No matter what if any backlash reducing system may be used its critical that you measure it. If its greater than 0.5mm you will have noticeable faults.

Craig
My wife left with my best friend...
     and I miss him!
Re: circles have a flat spot
« Reply #6 on: September 02, 2017, 08:31:31 PM »
Hi Frank,
you might be interested, I found this outfit and was impressed with prices:
http://www.automation-overstock.com/products.asp?cat=190

Craig
My wife left with my best friend...
     and I miss him!
Re: circles have a flat spot
« Reply #7 on: September 02, 2017, 08:53:25 PM »
Hi,
and for all you ever wanted to know, or even dreamed of about rack and pinion...
http://atlantadrives.com/systems.htm

I bought off Ebay an Atlanta Drive right angle servo reducer with the idea of making a fourth axis. Despite having the specs in front of me when I bought
it the physical reality of its size surprised me....'its big for the size of it!' and 'some surprises are surprisingly more surprising than other surprises!'. I haven't
yet got around to making my fourth axis yet, but I will one day. Its a beautifully built unit with only 2 arc backlash.

Craig
My wife left with my best friend...
     and I miss him!